1. Field of the Invention
The present invention relates to an apparatus for reproducing data employing a self-clock system such as an R-DAT (Rotary head system Digital Audio Tape recorder) and a digital video tape recorder, and more particularly to a data reproducing apparatus which is operated stably against bit rate variations at the time of searching and at a higher speed.
2. Description of the Prior Art
The digital recording technique has been applied to various fields such as sound and picture image recording in recent years, and techniques such as R-DAT and digital VTR techniques are being established. Among them, the technique of high speed searching for reading address information and control information while winding and moving a tape at a high speed has gained in importance along with a basic record reproducing technique.
Here, the outline of an R-DAT will be explained as a background technique. A track recorded obliquely on a tape with a rotary head of an R-DAT includes a plus azimuth track and a minus azimuth track. One track is composed of three data areas, a sub-data area 1, a main data area and sub-data area 2, and ATF areas 1 and 2 on which ATF signals used by tracking servos are recorded, etc. The sub-data areas 1 and 2 are areas where those data that are principally used for high speed searching such as time codes ae recorded, and are divided into 8 blocks, respectively, and block addresses are recorded at specific locations in the blocks. The main data area is an area where PCM audio data are recorded principally, and are divided into 128 blocks, and block addresses are recorded at specific locations in the blocks in the similar manner as the sub-data area. In respective data areas, block addresses are recorded at equal intervals.
The data that are base-band recorded on a tape medium are reproduced with two heads, one with a plus azimuth and one with a minus azimuth, opposedly disposed on a rotary cylinder. In general, the diameter of the cylinder is set at .phi.30 mm, the wrapping angle at 90 degrees, and the rotational speed at 2,000 rpm. The reproduced signal is amplified by a head amplifier and the waveform thereof is equalized with a equalizer so as to make it easy to extract data. In order to extract data, a clock corresponding to the bit rate of the reproduced signal is required. For this purpose, a clock reproducing phase locked loop circuit is employed. The reproduced data are supplied to a modulator-demodulator, applied with NRZI-NRZ conversion, 8-10 reverse conversion and deinterleaving here, and stored in a RAM once. Thereafter, error correction and error detection are executed by a code processor, and the reproduced data are supplied to a D/A converter through a PCM control portion and output as an analog signal output.
In regular reproducing, a recorded track is traced while keeping the tape travelling speed and the cylinder speed constant, thereby reproducing a clock of a constant bit rate so as to extract all of the data.
In searching, the tape is made to travel at a running speed several 100 times as high as that of regular reproducing. If the cylinder speed is also increased to several hundred times its normal speed, it is possible to trace the top of the recorded track, but the bit rate number is also increased to several hundred times its normal value due to the above noted state. Accordingly, the limit of achieving high speed in the circuit for reading and the limit within which the processor for processing may complete the operation within a predetermined period of time are far exceeded, and reproduction of the data can not be performed. It is not necessary to extract all of the data in searching, and it is only necessary to extract search control data. Thus, it is possible to partially read the data on the track recorded obliquely without changing the cylinder speed extremely from that of regular reproducing. In such a manner, it is arranged in an R-DAT so that the bit rate of a signal reproduced at the time of searching is almost the same as that of regular reproduction. That is, the track is traversed obliquely, but it becomes possible to keep the bit rate almost constant by controlling the relative velocity of the head to be almost constant.
Since the relative velocity of the head varies depending on the tape travelling speed and the cylinder speed, it is necessary to suppress such variation.
A technique for solving this problem has been disclosed in the prior art. As a conventional example, Japanese Laid-Open Patent Application No. 62-121951 may be considered. It is an object of this technique to increase reproducing reading capacity in high speed search. In order to achieve such an object, a mechanism control which makes the reproducing bit rate constant higher than a normal rate has been proposed. In this technique, a control signal is obtained by comparing the clock of the reproducing phase locked circuit obtained by F/V (frequency/voltage) conversion with a preset standard voltage. The bit rate is intended to be controlled to be almost constant by feeding back the control signal thus obtained to a servo at the cylinder speed. The problem of this technique exists in the forming method of the control signal, which results in a low reliability of the control signal. The R-DAT has two heads disposed opposedly on the cylinder, and winds the tape at a wrap angle of 90 degrees so as to thereby cause the tape to travel. Accordingly, even if the signals that are read from two heads are added together, still only intermittent data of a duty ratio of 1:1 are obtainable. Therefore, noises are mixed in sometimes during the time when data is lacking, viz., when the tape and the head are not contacted with each other opposedly, and the clock reproducing phase locked loop circuit causes unnecessary response because of noises in many occasions. It is not preferable to use the output of such a clock reproducing phase locked loop circuit as the control signal because of the low reliability thereof. There are stability problems such as a change with the passage of time and unevenness caused by analog processing when using an F/V conversion circuit.
Next, in order to allow bit rate variations to some extent and follow such variations, it is necessary for the capture range of the phase locked loop circuit to be wide.
As a conventional example, in Japanese Laid-open Patent Application No. 61-45451, a technique has been disclosed, in which when the locking of the phase locked loop circuit is released, detects such a release and has the range of the phase locked loop circuit displaced corresponding to the displaced quantity and follows such a displacement.
This technique presents a system for counting known data with a reproducing clock, which is able to generate control signals in accordance with the released quantity only when the locking of the phase locked loop circuit is released.
A first problem of this method exists in that data reproduction is interrupted until correction and redrawing are completed because displacement will not occur until the locking of the clock reproducing phase locked loop circuit is released. A second problem of this technique is that it is not possible at all to know the margin to the limit within the lock range because of the same reason as above, and therefore, it is not possible to prevent the release of locking from occurring. Moreover, a third problem of this technique is that detection reliability is insufficient, and confirmation thereof is impossible.
Furthermore, since data become intermittent at the time of searching, there is a problem related to the tape speed. In the R-DAT, data for searching such as program number data and address data are recorded on the same track as the principal data. As described above, the track is traversed obliquely at the time of searching. The speed of traversing the track (hereinafter referred to as the traverse speed) depends on the tape speed and is proportionate thereto in general. In the process of reproducing data at the time of traverse, on-track and off-track are repeated alternately. The readable data are those that exist during the time of on-track only. In other words, data reproduction is possible during the length of time in reverse proportion to the traverse speed. It is at least necessary that the data peculiar to the system required for search control appear in a group in the length of on-track time.
With the foregoing, in order to raise the limit of the tape speed at the time of searching, it becomes apparent in principle that it is more advantageous to increase the time density of the data.